You may need to write a PHP script that will write on the server.
Java->URL->PHP->TXT

It's unclear if this library is doing what you need, but it may be a good starting point: http://libraries.seltar.org/postToWeb/

Well, it depends if you have PHP running or not and what type of informations you want to add. This is a quick hack for small amount of information.

<onlineWriter.php>
Code:

<?php
	$data = $_GET['data'];
	$myFile = "output.txt";
	$fh = fopen($myFile, 'a') or die("can't open file");
	fwrite($fh, $data);
	fwrite("\n", $data);
	fclose($fh);
?> 

Then, you access your script through an URL request:
http://myServer.com/onlineWriter.php?data=SomeDataToAdd

You may use http://creativecomputing.cc/p5libs/prohtml/ to access the url.